Thermal control apparatus and method

ABSTRACT

The present invention provides a heating apparatus for heating a load. The heating apparatus comprises a heater having a heating element for receiving electrical power and for converting the electrical power into heat to heat a heating surface of the heater. The heating apparatus also comprises a temperature sensor for sensing and outputting a measurement of the temperature of the heating element, a power actuator for providing the electrical power to the heating element of the heater, a power sensor for sensing and outputting a measurement of the power provided to the heating element by the power actuator, and control circuitry for controlling the power actuator to control the power delivered by the power actuator to the heating element. The control circuitry is configured to receive the temperature measurement from the temperature sensor, receive the power measurement from the power sensor, combine the temperature measurement and the power measurement, and control the power actuator in dependence upon the combined temperature measurement and power measurement. This ensures that the temperature of the heating surface is constant throughout a period when the load is applied.

FIELD OF THE INVENTION

The present invention relates to an apparatus and method for controllingtemperature in heating devices. The invention has particular althoughnot exclusive relevance to an apparatus and method for controllingtemperature in devices for manipulating the shape of hair, for examplein order to style the hair.

BACKGROUND TO THE INVENTION

There are a variety of apparatus available for styling hair. One form ofapparatus is known as a hair straightener which employs plates that areheatable. To style, hair is clamped between the plates and heated aboveits glass transition temperature where the hair becomes mouldable.

A hair straightener device usually comprises a pair of arms hinged atone end and each arm comprising one or more heaters. Each heatertypically comprises a heating element, usually ceramic, and one or moreheating plates, usually aluminium. Such a device enables hair to besandwiched between the plates and heated from both sides. Hair has arelatively high thermal mass and when in contact with the heating platesthe hair absorbs a significant amount of the heat energy from theheating plates. The heating elements must quickly supply the lost heatenergy back to the heating plates otherwise the temperature of theheating plates will drop and potentially impact on the quality of thethermal styling. The thermal styling of hair relies on the glasstransition phase (GTP) temperature of hair: to style hair, the hair mustbe heated to above the GTP where the shape of the hair can then bemanipulated. When the hair cools back down, it will maintain that shapeuntil it is “reset” with water. If the temperature of the heating platesdrops such that the hair is not heated to above the GTP, then thestyling is poor. Also, if the temperature goes too high then the haircan be permanently damaged, hence it is important in such hair stylingdevices to have accurate temperature control. The temperature of theheating plates should be as high as possible to enable fast styling andthe hair heating up to the GTP as fast as possible, but not so hot thatit causes damage to the hair when the hair styling device is movedslowly through the hair.

Typically, hair straighteners are powered using high voltages (forexample 110Vac to 255Vac). This means the heating elements are poweredby high voltages (above Safety Extra Low Voltage, SELV), and where theelectronic circuitry is not isolated (for cost reasons), the temperaturesensor is attached to the electrically insulating ceramic heatingelement. Hence the temperature measured by the electronics via thesensor is not the actual temperature of the surface of the heating plate(or the surface of the hair) rather it measures the temperature of theceramic heating element. Due to the thermal resistance between theceramic heating element and the heating plate (and due to the thermalresistance of the heating plate and the ceramic heating elementthemselves) there is a temperature drop across the ceramic-aluminiumassembly which is proportional to the power provided by the heater, theproportionality constant being the thermal resistance.

The present invention seeks to provide a way of rapidly compensating forthe temperature drop across a heater that is thermally connected to aheater when a thermal load is suddenly applied.

SUMMARY OF THE INVENTION

The idea behind the novel control system described herein is to try toensure that the temperature of the heating surface is constantthroughout the period when the load is applied. The invention can beapplied to a wide variety of devices including hair straighteners orhair stylers, cooking appliances, heated mixing bowls, water heaters,coffee makers as well as other hair styling devices such as curlingtongs, curling devices, eye lash curlers and hair dryers. The inventionallows water to be heated in a fast and efficient manner, for examplewhen used in coffee making devices, kettles and showers.

In a first aspect, the present invention provides heating apparatus forheating a load, the heating apparatus comprising: a heater having aheating element for receiving electrical power and for converting theelectrical power into heat to heat a heating surface of the heater; atemperature sensor for sensing and outputting a measurement of thetemperature (T_(D)) of the heating element; a power actuator forproviding the electrical power to the heating element of the heater; apower sensor for sensing and outputting a measurement of the powerprovided to the heating element by the power actuator; and controlcircuitry for controlling the power actuator to control the powerdelivered by the power actuator to the heating element, wherein thecontrol circuitry is configured to: receive the temperature measurement(T_(D)) from the temperature sensor; receive the power measurement (P)from the power sensor; combine the temperature measurement and the powermeasurement; and control the power actuator in dependence upon thecombined temperature measurement and power measurement.

The control circuitry may be further configured to: determine a newoutput power (P_(new)) based on a target temperature (T_(D)) of theheating surface and a temperature differential (ΔT) in the heater;wherein the temperature differential (ΔT) is based on the received powermeasurement (P) and a thermal resistance (R_(th)) of the heater.

The temperature differential (ΔT) in the heater may be calculated basedon the received power measurement (P) multiplied by the thermalresistance (R_(th)) of the heater.

Determining a new output power (P_(new)) may include: determining adifference (D₁) between the received temperature measurement (T_(D)) andthe target temperature (T_(D)) of the heating surface.

Determining a new output power (P_(new)) may further include: adding thetemperature differential (ΔT) in the heater to the determined difference(D₁) between the received temperature measurement (T_(C)) and the targettemperature (T_(D)) of the heating surface.

The control circuitry may be further configured to determine the newoutput power (P_(new)) such that T_(C)−T_(D) P_(new)R_(th)=0.

The heater may further comprise an electrically-insulating interfacebetween the heating element and the heating surface.

The heating surface may be metallic.

The heating apparatus may comprise a hair styler.

The hair styler may comprise first and second mutually-opposing armsadapted for movement between an open configuration for receiving alength of hair therebetween and a closed configuration in which hair issandwiched between the opposing arms, and at least one of the arms mayinclude the heater.

The control circuitry may be further configured to apply a weighting tothe power measurement (P) before combining the temperature measurementand the weighted power measurement.

The weighting may be chosen so that the weighted power measurementrepresents a temperature drop between the temperature of the heatingelement and the temperature of the heating surface.

The control circuitry may be further configured to combine thetemperature measurement and the power measurement such that the combinedtemperature measurement and power measurement represents a measure ofthe temperature of the heating surface.

The control circuitry may be further configured to compare thetemperature of the heating surface with a desired temperature of theheating surface to generate an error signal and to control the poweractuator to reduce the error signal.

The control circuitry may be further configured to filter thetemperature measurement in order to reduce noise and/or configured tofilter the power measurement to reduce noise.

In another aspect, the present invention provides a method ofcontrolling a heater, the heater comprising a heating element forreceiving electrical power from a power actuator and for converting theelectrical power into heat to heat a heating surface of the heater, saidmethod comprising: receiving a temperature measurement (T_(D)) of theheating element; receiving a power measurement (P) of the electricalpower provided to the heating element; combining the temperaturemeasurement and the power measurement; and controlling the poweractuator in dependence upon the combined temperature measurement andpower measurement.

In another aspect, the present invention provides a hair stylercomprising: a heater for heating hair, the heater having a heatingelement for receiving electrical power and for converting the electricalpower into heat to heat a heating surface of the heater; a temperaturesensor for sensing and outputting a measurement of the temperature(T_(C)) of the heating element; a power actuator for providing theelectrical power to the heating element of the heater; a power sensorfor sensing and outputting a measurement of the power provided to theheating element by the power actuator; and control circuitry forcontrolling the power actuator to control the power delivered by thepower actuator to the heating element, wherein the control circuitry isconfigured to: receive the temperature measurement (T_(C)) from thetemperature sensor; receive the power measurement (P) from the powersensor; combine the temperature measurement and the power measurement;and control the power actuator in dependence upon the combinedtemperature measurement and power measurement.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, and with reference to the drawings in which:

FIG. 1 illustrates a hair styler;

FIG. 2 is a cross-sectional view through a heater;

FIG. 3 is a schematic block diagram showing a thermal control system;

FIG. 4 is a schematic block diagram showing the main components ofcontrol electronics forming part of the system shown in FIG. 3;

FIG. 5 is an equivalent electrical circuit of the control electronics;

FIG. 6 is a plot showing experimental results obtained using anexemplary implementation; and

FIG. 7 is a schematic block diagram showing another thermal controlsystem.

In the Figures, like elements are indicated by like reference numeralsthroughout.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present embodiments represent the best ways known to the applicantsof putting the invention into practice. However, they are not the onlyways in which this can be achieved.

Overview of Hair Styler

FIG. 1 illustrates a hair styler 1 which employs a thermal controlsystem. The hair styler 1 includes a first movable arm 4 a and a secondmovable arm 4 b, which are coupled together by a hinge mechanism 2. Thefirst and second movable arms 4 a, 4 b oppose one another and aremovable relative to one other by virtue of the hinge mechanism 2. Thus,the first and second arms 4 a, 4 b can be brought together, into aclosed configuration, or moved apart, into an open configuration, by auser during use.

The first arm 4 a bears a first heating plate 6 a, and the second arm 4b bears a second heating plate 6 b. The first and second heating plates6 a, 6 b oppose one another and, in use, are brought together as thefirst and second arms 4 a, 4 b are brought together, or separated as thefirst and second arms 4 a, 4 b are moved apart. The heating plates 6 a,6 b form part of respective heaters.

The hinge mechanism 2 can incorporate any suitable means for allowingthe first and second arms 4 a, 4 b to be moved relative to one other.

The hinge mechanism 2 may be configured to bias the first and secondarms 4 a, 4 b into the open configuration, such that the user isrequired to apply pressure to the arms 4 a, 4 b to close them together,and such that the arms 4 a, 4 b automatically open once the pressure isremoved. For example, the hinge mechanism 2 may incorporate a leafspring or a coiled spring to provide this bias. Alternatively oradditionally, the first and second arms 4 a, 4 b may be formed in aunitary manner (e.g. from a plastics material) with a “U” shaped middlepart provided between the first and second arms 4 a, 4 b, the “U” shapedmiddle part being able to resiliently flex to allow opening and closingof the first and second arms 4 a, 4 b.

The electrical and electronic circuitry of the hair styler 1 is housedin the first arm 4 a, with a switch 3 being provided on the first arm 4a to enable the styler 1 to be turned on or off, together with a light 5to indicate whether the power is on. The electrical and electroniccircuitry of the hair styler 1 could also be housed in the second arm 4b, or split between arms 4 a and 4 b. A sound can also be played by asound generator (not illustrated) when the styler 1 is switched on andready to use. Together, the switch 3, light 5 and sound generator (ifincluded) form a user interface. In alternative embodiments the userinterface may include additional components (such as, for example, adisplay means, to provide the user with more information on theoperational status of the styler).

In use, hair is clamped between the two heating plates 6 a, 6 b and thenthe styler is moved such that the hair is pulled through the stylerbetween the first and second heating plates 6 a, 6 b. Heat passes fromhair contacting surfaces of the heating plates 6 into the hair to allowthe styling of the user's hair using the device.

Heater

FIG. 2 is a cross-sectional view through the heater 10 (across the widthof the arm 4) that shows the main components of the heater 10 (alsoreferred to as a heater stack or a thermal stack) carried on each arm 4.As shown, the heater 10 includes a heating element 15 and a heatingplate 6 which are thermally connected via an interface 18.

Typically, the heating element 15 is formed from a ceramic material andthe heating plate 6 is formed from aluminium.

The interface 18 typically comprises a thermal paste which is thermallyconducting, for example aluminium oxide paste.

The interface 18 may additionally or alternatively comprise a thermalbarrier. This may comprise a layer of tightly sintered ceramic and/or alayer of a thermally stable plastic, such as Kapton tape. Typically,such a barrier will be present in the heater 10 in order to provideelectrical insulation for improved safety, or to conform to standards orregulations. As a specific example, some territories have regulationswhich require electrical products such as hair stylers to include alayer of Kapton tape between electrical heating means and externalsurfaces for safety purposes (e.g. to reduce the risk of the userreceiving an electrical shock). Most electrical insulators are excellentthermal insulators and hence have very high thermal resistances.

The heater 10 also includes a temperature sensor 19 (such as athermistor) which is disposed on the heating element 15, on an opposingside to the heating plate 6.

FIG. 2 also illustrates some parameters of the heater including: T_(p)which is the temperature of the heating plate 6; T_(c) which is thetemperature of the heating element 15 as measured by the temperaturesensor 19; ΔT which is the temperature drop between the heating element15 and the heating plate 6; and R_(th) which is the thermal resistancebetween the heating element 15 and the heating plate 6. Preferably,R_(th) takes into consideration the thermal resistance of the heatingplate 6 and the thermal resistance of the heating element 15. ΔT is thusthe temperature difference across this thermal resistance.

Using standard laws of physics, the plate temperature can be calculatedas:

T _(P) =T _(C) −ΔT  Equation 1

ΔT is related to the thermal resistance and thermal power, P_(I),crossing the thermal interface 18, so T_(P) can be written as:

T _(P) =T _(C) −R _(th) P _(I)  Equation 2

Accordingly, if the power P_(I) crossing the interface 18 and thethermal resistance R_(th) of the interface 18 are measured (or known),then the temperature T_(P) of the heating plate 6 can be determined.

In Equation 2, power P_(I) is a thermal power—i.e. Q/t where Q is thethermal energy in joules and t is time. The thermal energy Q originatesfrom the heating element 15, which coverts electrical energy into heatenergy. As described below, the heating element 15 is supplied with anelectrical power P. The inventors identified that the losses in thesystem in converting the electrical power into thermal power are smalland generally constant, and therefore measurement of the electricalpower P supplied to the heating element 15 can be used to infer thethermal power P_(I) dispersed across the interface 18. This can be done,for example, by applying a suitable weighting factor to electrical powerP in order to take into account system losses, or by incorporating theweighting factor into the thermal resistance R_(th) of the interface 18.Typically, the weighting factor is a constant multiplicative factor.

In prior art systems, feedback control loops act in such a manner thatthe heating element's temperature is kept as constant as possible, byturning on and off the heater through a power actuator. In other words,the control loop acts to ensure T_(C) and the desired plate temperatureor target temperature, T_(d), are as close as possible. This means thatT_(P) will always be less than T_(C) due to Equation 2: P_(I) and R_(th)are always positive, hence the temperature might not be high enough tostyle.

Also, it is desirable to have the heating plate 6 (which contacts thehair) to remain at a constant desired (target) temperature. This couldbe achieved by placing a sensor in the heating plate 6 or very near tothe surface of the heating plate 6 which makes contact with the hair.However, this is expensive and on high voltage systems very difficult,because there needs to be an isolation clearance distance between theconductive parts which can be touched by the user and the electriccircuit. Using electrically insulating material is possible, but usuallysuch material is an excellent thermal insulator and hence you have thesame problem described above.

As will be described in more detail below, the control electronics 24use knowledge of the styling device to determine T_(P) based uponfluctuations of T_(C) and P and then adjusts P to ensure a constantT_(P).

In the prior art, thermal resistances between a heating element and aheating plate can cause performance degradation during styling of hairbecause the time taken for the temperature sensor to recognise atemperature drop on the plate is long, therefore increasing themagnitude of the temperature drop. The present invention uses thefluctuations of T_(C) and P to calculate a control signal, the controlsystem is employing a type of feed forward control which advantageouslyensures a constant T_(P), even if a thermal barrier exists between theheating element 15 and heating plate 6 (e.g. ΔT is high), and even if aload is applied to the heating plate 6.

Thermal Control System

FIG. 3 is a schematic block diagram showing a thermal control system 20used in the hair styler 1 of FIG. 1 to control the delivery ofelectrical power to a heater 10.

The thermal control system 20 includes a power supply unit 21, controlelectronics 24, a power actuator 22, a user interface 25, a power sensor27, a heating element 15 and the temperature sensor 19. Forcompleteness, FIG. 3 also shows the heating plate 6. The thermal controlsystem 20 is configured to vary the electrical power supplied to theheating element 15 in order to maintain the heating plate 6 at aconstant temperature T_(P) when a thermal load (such as the user's hair)is applied to the heating plate 6.

The heating element 15 is thermally coupled to the temperature sensor19, which provides measurement of the temperature T_(C) of the heatingelement 15.

The power actuator 22 powers the heating element 15. The power actuator22 could include a triac and/or any suitable means for controlling poweroutput to the heating element 15.

The power sensor 27 measures the electrical power P output by the poweractuator 22, and provides this measurement of power P to the controlelectronics 24. As stated above, since there are few losses in thesystem, the electrical power supplied to the heating element 18 can beequated to the thermal power P_(I) crossing the interface 18 between theheating element 15 and the heating plate 6, and the weighting factorrepresenting loss of energy in the system is included the thermalresistance R_(th) of the interface 18. This is done during calibrationof the system, as explained further below.

As described above with reference to equation 2, the temperature T_(P)of the heating plate can be determined from the temperature of theheating element 15 (T_(C)) and by measuring the power P_(I) crossing theinterface 18 between the heating element 15 and the heating plate 6 andthe thermal resistance R_(th) of the interface 18.

The control electronics 24 (which may include a PID (proportional,integral, derivative) controller) control the power actuator 22 based onboth the measurement (TO of the temperature of the heating element 15received from the temperature sensor 19 and the measurement (P) of thepower output by the power actuator 22 received from the power sensor 27.These measurements are used by the control electronics 24 to control thepower output by the power actuator 22 in order to try and reach a targettemperature T_(d) of the heating plate 6 defined by the user via theuser interface 25. T_(d) is also referred to as the set-point or controltemperature and preferably has a value of about 185° C.

Although 185° C. is the preferred target temperature, for styling hairthe target temperature can range from, for example, 30° C. to 230° C.This advantageously allows for a variety of styling options, including“wet to style” where the hair styler is applied to wet hair. In thiscase, the water content of the hair can be measured, for example byusing a detector which compares the amount of radiant energy in twoabsorption bands in the spectrum of light emitted by an infra-red sourceand reflected by the hair. Based on this measurement, the targettemperature can be adjusted accordingly to stop damage occurring. Asdescribed further below, other ranges of target temperature may be usedfor different applications, such as in electric showers.

As will be described in more detail below, the control electronics 24are effectively configured to use the measured values of the heatertemperature T_(C) and electrical power P and pre-stored data relating tothe thermal resistance of the interface 18 (R_(th)), to determine theactual value of T_(P), and from this an error between the target valueT_(d) and actual value T_(P). The control electronics 24 then uses thiserror to calculate and output a control signal that will cause the poweractuator 22 to output an updated electrical power signal that is appliedto the heating element 15 and that will reduce this error to zero.

The power actuator 22 can vary the power delivered to the heatingelement 15 either by varying the current and/or the voltage applied tothe heating element 15. In a preferred embodiment, the control signalgenerated by the controller is a pulse width modulated (PWM) signal thatswitches the power actuator 22 on and off, and wherein the power isvaried by varying the mark-space ratio of the PWM control signal.

The pre-stored data relating to the thermal resistance of the interface18 (R_(th)), is stored in non-volatile memory (not illustrated) formingpart of the control electronics 24. This is typically done when thestyling device is first calibrated during production, where R_(th) andthe electrical resistance of the heater 10 are measured. In particular,this is achieved in production by using external apparatus to controlthe device via a serial communications link and set T_(C) on the deviceto a known target value. Then both T_(C) and T_(P) and their respectiverates of change are measured directly using the external apparatus. Thisdata can then be used to calculate the actual values of heaterelectrical and thermal resistance. These values are then loaded into thenon-volatile memory, e.g. via a communications link. At this point, theenergy losses in the system can also be measured and accounted for, byincorporating a weighting factor into the thermal resistance R_(th).Alternatively or additionally, the system losses can be accounted for inthe control electronics 24 by applying a suitable weighting factor tothe measured electrical power P.

R_(th) is the thermal resistance of the load or plate to air—it isbelieved that this resistance is unlikely to change between the platebeing loaded or unloaded (hair being present or not). This is obviouslya simplified system and does not consider thermal masses. Practicallyand against standard thinking, the inventors have found that the thermalmasses do not need to be considered.

The measurement signals generated by both the power sensor 27 and thetemperature sensor 19 may be passed through electrical filters to reducethe impact of noise. Likewise, the signal produced by the power actuator22 can also be filtered to reduce sudden changes in the electrical powerapplied to the heating element 15.

The power supply unit 21 provides power to the thermal control system20. In this embodiment, the power supply unit 21 is connected thecontrol electronics 24, which in turn provides power to the poweractuator 22 which powers the heating element 15. It will be appreciatedthat different arrangements may be provided for powering the thermalcontrol system 20, for example by connecting the power supply unit 21directly to the power actuator 22.

The power supply unit preferably powers the thermal control system 20using relatively high voltages (for example 110Vac to 255Vac). Forexample, the power supply unit 21 may supply mains voltage (230Vac) tothe thermal control system 20.

Control Loop

FIG. 4 is a schematic block diagram illustrating the main components ofthe control electronics 24 forming part of the control system shown inFIG. 3. As shown, the temperature measurement T_(C) is input to a filterblock 31 that filters the temperature measurement (to reduce the effectsof noise). The filter block 31 is represented by a transfer function(E). The filtered temperature measurement is then subtracted from thedesired or set point temperature T_(d) in the subtracting unit 33.Similarly, the power measurement (P) is filtered in filter block 35 toreduce the effects of noise. The filter block 35 is represented by atransfer function (H). The filter block 35 has a gain corresponding toR_(th) (the thermal resistance between the heating element 15 and theheating plate 6—as determined for the styling device during the abovedescribed calibration routine). The weighted and filtered powermeasurement is then input to the adding unit 37 where it is added to theoutput from the subtracting unit 33. The output or error signal (e) fromthe adding unit 37 is then input to the controller 39 and is given by:

e=T _(d) −T _(C) +R _(th) P=T _(d)−(T _(C) −R _(th) P)=T _(d) −T_(p)  Equation 3

As T_(d) is the desired temperature for the heating plate 15 and T_(p)is the actual heating plate temperature, the error signal thus generatedcan be used by the controller 39 to control the power applied to theheating element 15 in order to drive this error signal to zero. Thecontroller 39 can calculate an update to the output control signal asoften as new measurements become available from the sensors.Alternatively, the filters may filter multiple measurements beforeoutputting a filtered value for the controller 39. The inventors havefound that the controller only needs to update the output control signalat a rate of about once every 0.1 to 0.5 seconds to achieve a steadycontrol for the hair styler.

As discussed above, in the preferred embodiment, the control signalgenerated by the controller is a pulse width modulated (PWM) signal thatswitches the power actuator 22 on and off, and wherein the power isvaried by varying the duty ratio of the PWM control signal. Care mustalso be taken to ensure that the controller 39 does not try to drive theheater with a power that is greater than the rated power of the heater(as defined by the heater manufacturer as P_(max)). The controller 39does this by ensuring the duty ratio of the PWM control signal does notexceed a maximum value (D_(max)) calculated from:

$D_{\max} = {\frac{P\max*R}{V^{2}} = \frac{P\max}{Pattainable}}$

Where V is the voltage received from the power supply unit, e.g. 230Vacand R is the electrical resistance of the heater.

Transfer Function Representation

FIG. 5 is an electrical equivalent circuit of the control loop,illustrating the temperature and power sensors, the filters and thecontroller as blocks having transfer functions. The load on the heater10 is represented by the resistor R_(load) and the thermal resistancebetween the heating element and the heating plate is represented by theresistor R_(th).

The combined transfer functions of the temperature sensor and filter,the power sensor and filter, and the controller and power actuator areE, H and G respectively.

As shown in FIG. 5, temperature measurement T_(C) is filtered to providea filtered temperature measurement T_(sf), where T_(sf)=E*T_(c).

Power measurement P is filtered to provide a filtered power measurementP_(sf), where P_(sf)=H*P.

Preferably, temperature is measured using tenths of a degree and poweris measured using milliwatts.

The ratio of target temperature T_(d) to plate temperature T_(P) cantherefore be given by:

$\frac{Tp}{Td} = \frac{1}{{\frac{1}{Rload}*( {\frac{1}{G} - H} )} + {E*( {1 + \frac{Rth}{Rload}} )}}$

If the gain of the transfer function H is set as:

H = E * Rth Then:$\frac{Tp}{Td} = {\frac{1}{E}*\frac{1}{\frac{1}{G*{Rload}} + 1}}$

In practice, in the case of a hair straightener where the plates arebeing pulled through hair, the relevant input (i.e. input excludingnoise and undesirable signals) is constant or DC and hence the transferfunctions can be defined by pure real gains (not complex), andG*R_(load) is very large for all R_(load) values of interest. Thismeans:

$\frac{Tp}{Td} \approx \frac{1}{E}$

Hence, if the gain of the transfer function E is unity, then the steadystate value of the plate temperature (T_(P)) is the target temperature(T_(d)), irrespective of the load. Thus using the feedback scheme inFIG. 5, the heater plate temperature (T_(P)) can be accurately trackedto the target temperature without the need for a sensor on the heaterplate.

Normally the filters 31 and 35 are chosen to reduce noise (e.g. 50 Hznotch filter for the temperature sensor) or average the signals (e.g.low pass filter for the power sensor, since the nature of controlling ACpower using semi-cycles is not very smooth).

The filter transfer functions can also be adjusted to change the dynamicresponse of the system and can include complex terms to change phaseetc.

Consideration of the stability of the control system must also be madein the choice of filter transfer functions. As a safety precaution inour implementation, the stability was checked at runtime by ensuringthat if the output of the controller 39 is zero, then the temperatureshould stop increasing within a predefined timeout.

Experimental Results

FIG. 6 is a plot showing results obtained using an exemplaryimplementation. In this implementation, the heating plate temperatureT_(p) was measured directly.

In the exemplary implementation, the transfer functions are described asfollows:

E is a 2nd order notch filter with unity gain except in the stopband(30-80 Hz).

G has a value 15, consisting of a PID controller (P=1.5, I=0, D=0) and apure cascade gain of 10.

H is a 1st order low pass filter whose gain is set to Rth as measuredduring the calibration routine described above (typically 0.133 in ourembodiment).

The design of the exemplary implementation is such that when the heatingplates are idling (not heating hair) and the power actuator 22 isdelivering 12 W to the heating plates 15, Rload computes to be 15 ohms,therefore G*Rload=225, its inverse 0.004, and thus Tp/Td=0.996. When theheating plates are loaded with hair and the power actuator 22 isdelivering 30 W to the heating plates 15, Rload is 5 ohms, G*Rload=75,its inverse 0.013, and thus Tp/Td=0.987.

Because of the way the exemplary implementation was designed,controlling power delivery to the heaters every 320 msec, there is alsoa time delay. In reality, Rload is actually part of a complex impedanceZload consisting of Rload and Cload in parallel. In the steady or DCstate the frequency dependent terms in all the transfer functions arezero, and we may consider the above transfer functions to be pure gains.

In FIG. 6 the y axis shows the temperature in degrees Celsius of theheating plate Tc and the temperature of the plate T_(P) and the x axisshows the time in seconds. The results are summarised as follows.

0-119 sec-no control based on T_(C) and P:

Between 0 and 30 sec the heating plates 15 were unloaded (no hair waspresent between the heating plates 15): It can be seen that T_(c) is atthe set-point temperature T_(d)=185° C. However the plate temperatureT_(p) is at 183° C., 2° C. below the T_(c). Power P was measured to be20 W. A load (hair) was applied between the heating plates 15 at t=30sec for a duration of 20 sec. As can be seen the plate temperature T_(p)drops from 183° C. to 178° C. There is an apparent slight increase inT_(c), but this is due to the localised nature of the load and the spotnature of the measurement thermocouple (the controller 39, because ituses an averaging sensor, sees no temperature change in T_(c))

120-300 Sec-Control Based on T_(C) and P

At t=120 sec, control based on T_(C) and P was activated with theparameters 2° C. temperature drop and 20 W power. Immediately, the platetemperature T_(p) went up to 185° C. (the same as the set-pointtemperature T_(d)), and T_(c) went up to 187° C. At t=200 sec, the sameload as before was applied for a duration of 40 sec. The marked rise inT_(c) to over 200° C. is due to the above control action, and thisresults in the plate temperature staying almost the same (the rise of 2°C. can be removed by finer tuning of the compensation parameters). Itcan be seen that as the load temperature rises, the temperature T_(c)begins to fall, which is consistent with less power being drawn. At timet=240 seconds the load was suddenly removed, and the system returned toits undisturbed state.

As can be seen from the above, by measuring the power applied to theheating element and using it to effectively determine a measure of thetemperature drop between the heating element 15 and the heating plate 6,the controller 39 is able to quickly compensate for changes to thethermal loading applied to the heating plate 6. This allows for betterand more accurate control of the heater plate 6.

Additional Heaters

The above description explains the control strategy for controlling thetemperature of a single heater plate. As discussed above in the hairstyler of FIG. 1, there are two heaters (heating elements and heatingplates). Parallel control loops may be provided for controlling the twoheaters independently. Alternatively, one controller may be provided tocontrol the heating of both heaters—as illustrated in FIG. 7.

As shown in FIG. 7, a thermal control system 70 includes a power supplyunit 21, control electronics 24, a power actuator 22 and a userinterface 25. These elements are similar to the elements described abovein FIG. 3, except that the control electronics 24 and power actuator 22of thermal control system 70 are adapted to control the heating of twoheaters. Heating element 15 a, temperature sensor 19 a, and heatingplate 6 a form part of a first heater, and heating element 15 b,temperature sensor 19 b, and heating plate 6 b form part of a secondheater.

A first power sensor 27 a measures the electrical power P output by thepower actuator 22 to the first heater, and provides this measurement ofpower P to the control electronics 24. A second power sensor 27 bmeasures the electrical power P′ output by the power actuator 22 to thesecond heater, and provides this measurement of power P′ to the controlelectronics 24.

The thermal control system 70 is configured to vary the electrical powersupplied to the heating elements 15 a and 15 b in order to maintain theheating plates 6 a and 6 b at a constant temperature T_(P) when athermal load (such as the user's hair) is applied to the heating plates6 a and 6 b.

MODIFICATIONS AND ALTERNATIVES

An embodiment has been described above illustrating the way in whichcontrol electronics can determine the temperature of a heating surface(the surface of the heating plate) without using a temperature sensor onthe plate—but by measuring the power drawn by the device as a load isapplied to the heating surface and relating that to a temperature dropbetween the heating element whose temperature is being sensed and athermal resistance between the heating element and the heating plate(determined during a calibration routine).

As those skilled in the art will appreciate, various modifications andalternatives can be made to the above described control system and someof these will now be described.

In the heater described above, the temperature sensor was attached to aside of the heating element that was opposite to the heating plate. Inan alternative embodiment, the temperature sensor may be embedded withinthe heating element—so that the temperature sensor senses thetemperature of the heating element at a position closer to the heatingplate. Furthermore, the temperature sensor may not be part of the heateritself but may be attached to and/or placed in thermal contact with theheater, and preferably in contact with the heating element.

In the heater described above, a thermal interface was provided betweenthe heating element and the heating plate. The thermal interface maycomprise one or more of a thermal paste, a thermal barrier/insulator, anair gap, and an adhesive. This thermal interface may be omitted ifdesired—in which case the heating element will be in direct contact withthe heating plate. However, the use of a thermal interface is preferredas it allows for a better thermal contact between the heating elementand the heater plate which allows for better heat transfer between them.

In the above embodiments the power output by the power actuator 22 ismeasured using a power sensor. There are various ways of measuring theelectrical power output by the power actuator 22 to the heater. Forexample, the voltage or current applied to the heater can be measuredfrom which a power measure can be determined using the conventionalP=I²R or P=V²/R equations.

In the case that the duty cycle of the voltage (or current) is beingvaried by the power actuator, the root mean squared (rms) voltage (orcurrent) may be measured and multiplied by the duty cycle of the poweractuator. Thus V²=Vrms²×duty.

The electrical resistance of the heater (R) depends on the temperatureof the heater. The temperature/resistance curve is defined by themanufacturer and so the power measurement can be determined by using thesensed temperature to determine the heater resistance and multiplyingthis with the value of V² calculated above.

The control electronics can also be configured to control the maximumpower output of the power actuator, in order to vary the amount ofvolume acquired by hair when being styled. Due to the dynamic responseof hair, a high maximum power means that, when a tress of hair isapplied to the heating plate, the hair will be heated quickly, whichresults in lower-volume, straight hair. On the other hand, a lowermaximum power means that when a tress of hair is applied to the heatingplate, the hair will be heated more slowly, which results inhigher-volume hair.

It is noted that, for large thermal loads, reducing the maximum powerwill likely lower the rise in temperature of the load for theapplication period overall. However, for smaller thermal loads such ashair, the overall rise in temperature is approximately the same, whilethe rate of heating varies. It is the rate of heating which affects thehair volume.

In the above embodiment, the heater included a heating plate. As thoseskilled in the art will appreciate, the heating plate does not need tobe in the shape of a plate. The purpose of the heating plate is toprovide a heating surface from which heat can pass from the heatingplate to the load to be heated—in the above case the hair to be heated.That heating surface can have any shape.

The values given in the transfer functions described above areexemplary. Those skilled in the art will appreciate that these valuescan be varied depending on the particular application, for example indevices other than hair stylers.

A new control system has been described for controlling the applicationof power to the heaters of a hair straightener. As those skilled in theart will appreciate, the techniques described above may be employed in awide range of other hair styling and/or manipulation appliancesincluding, but not limited to a hair crimping device, eyelash curlers,and a hair curler.

Furthermore, the techniques described above may be applied to many otherdevices employing thermal control, for example electric water showers,cooking devices and coffee making machines.

In the case of an electric shower, with a system which is thermallycontrolled in the standard manner, it can take a few minutes for thewater to heat up to the desired temperature, and the water may notremain at the desired temperature in certain circumstances. For example,when the shower is turned off, the residual heat in the heating elementcan heat the water in the heating area to an undesirable temperature. Ifthe shower is then suddenly turned back on again, this slug of water caneasily scold the user.

Using the techniques described above, these problems can be at leastpartially addressed. By measuring the temperature of the heating elementand the power being supplied to the heating element by a power actuatorin the shower (e.g. the power consumption of the shower), the poweractuator can be controlled to reduce the period of time taken for thewater to reach the desired temperature. Also, by measuring heatingelement temperature and power input, the residual heat in the heater canbe reduced, because the control of the temperature of the heater is moreaccurate. The control can also be made even more accurate by measuringwater flow through the shower, as this indicates the load being appliedto the heater.

For the electric shower, the control temperature is preferably in therange of 60° C. to 95° C.

In the case of a coffee making machine, it is important that the steamor hot water being applied to the ground beans is at a constanttemperature and the correct temperature to ensure the correct organicchemicals from the beans end up in the coffee to ensure the correctbitterness and level of crema. In the past, this was traditionallyachieved using a large copper block to store the heat energy to heat thewater. This means such coffee making machines require a long time towarm up and be ready for operation, and the machines are heavy andinefficient. A cheaper and more convenient alternative is to use highpower heating elements, but these must be accurately controlled toensure the correct water or steam temperature.

Using the techniques described above, these problems in coffee makingmachines can be at least partially addressed. By measuring thetemperature of the heating element and the power being supplied to theheating element by a power actuator in the coffee making machine (e.g.the power consumption of the coffee making machine), the watertemperature can be accurately controlled. The control could be furtherimproved by additionally measuring water flow and the ground beantemperature.

For the coffee making machine, the control temperature is preferably inthe range of 80° C. to 120° C.

In the case of a heated mixing bowl or similar cooking device (examplesof such product include Vorwerk Thermomix and Kenwood food mixer), it isimportant to ensure that the food being mixed is not over heated and isevenly heated to prevent lumpy sauces or burnt food. Using thetechniques described above, by measuring the temperature of the heatingelement and the power being supplied to the heating element by a poweractuator in the heated mixing bowl or similar cooking device (e.g. thepower consumption of the device), the heat applied to the food can beaccurately controlled. Similarly, this could apply to ovens and sousvide devices.

For the cooking devices, including heated mixing bowls and ovens, thecontrol temperature is preferably in the range of 30° C. to 230° C.

No doubt many other effective alternatives will occur to the skilledperson. It will be understood that the invention is not limited to thedescribed embodiments and encompasses modifications apparent to thoseskilled in the art lying within the scope of the claims appended hereto.

Throughout the description and claims of this specification, the words“comprise” and “contain” and variations of the words, for example“comprising” and “containing”, means “including but not limited to”, andis not intended to (and does not) exclude other components, integers orsteps.

1. Heating apparatus for heating a load, the heating apparatuscomprising: a heater having a heating element for receiving electricalpower and for converting the electrical power into heat to heat aheating surface of the heater; a temperature sensor for sensing andoutputting a measurement of the temperature (T_(C)) of the heatingelement; a power actuator for providing the electrical power to theheating element of the heater; a power sensor for sensing and outputtinga measurement of the power provided to the heating element by the poweractuator; and control circuitry for controlling the power actuator tocontrol the power delivered by the power actuator to the heatingelement, wherein the control circuitry is configured to: receive thetemperature measurement (T_(C)) from the temperature sensor; receive thepower measurement (P) from the power sensor; combine the temperaturemeasurement and the power measurement; and vary the power delivered tothe heating element by the power actuator in dependence upon thecombined temperature measurement and power measurement to drive thetemperature of the heating surface to a desired operating temperature.2. Heating apparatus according to claim 1, wherein the control circuitryis configured to determine a measure of the temperature of the heatingsurface by combining the temperature measurement and the powermeasurement and to control the power actuator in dependence upon thedetermined measure of the temperature of the heating surface
 3. Heatingapparatus according to claim 1, wherein the control circuitry is furtherconfigured to: determine a new output power (P_(new)) based on a targettemperature (T_(D)) of the heating surface and a temperaturedifferential (ΔT) in the heater; wherein the temperature differential(ΔT) is based on the received power measurement (P) and a thermalresistance (R_(th)) of the heater.
 4. Heating apparatus according toclaim 3, wherein temperature differential (ΔT) in the heater iscalculated based on the received power measurement (P) multiplied by thethermal resistance (R_(th)) of the heater.
 5. Heating apparatusaccording to claim 3, wherein determining a new output power (P_(new))includes: determining a difference (D₁) between the received temperaturemeasurement (T_(C)) and the target temperature (T_(D)) of the heatingsurface.
 6. Heating apparatus according to claim 5, wherein determininga new output power (P_(new)) further includes: adding the temperaturedifferential (ΔT) in the heater to the determined difference (D₁)between the received temperature measurement (T_(C)) and the targettemperature (T_(D)) of the heating surface.
 7. Heating apparatusaccording to claim 1, wherein the control circuitry is furtherconfigured to determine the new output power (P_(new)) such thatT_(C)−T_(D)−P_(new) R_(th)=0.
 8. Heating apparatus according to claim 1,wherein the heater further comprises an interface between the heatingelement and the heating surface.
 9. Heating apparatus according to claim8, wherein the interface comprises at least one of anelectrically-insulating interface and a thermally-insulating interface.10. Heating apparatus according to claim 1, wherein the heating surfaceis metallic.
 11. Heating apparatus according to claim 1, wherein theheating apparatus comprises a hair styler.
 12. Heating apparatusaccording to claim 11, wherein the hair styler comprises first andsecond mutually-opposing arms adapted for movement between an openconfiguration for receiving a length of hair therebetween and a closedconfiguration in which hair is sandwiched between the opposing arms, andat least one of the arms includes the heater.
 13. Heating apparatusaccording to claim 1, wherein the control circuitry is furtherconfigured to apply a weighting to the power measurement (P) beforecombining the temperature measurement and the weighted powermeasurement.
 14. Heating apparatus according to claim 13, wherein theweighting is chosen so that the weighted power measurement represents atemperature drop between the temperature of the heating element and thetemperature of the heating surface.
 15. Heating apparatus according toclaim 2, wherein the control circuitry is further configured to comparethe temperature of the heating surface with a desired temperature of theheating surface to generate an error signal and to control the poweractuator to reduce the error signal.
 16. Heating apparatus according toclaim 1, wherein the control circuitry is further configured to filterthe temperature measurement in order to reduce noise and/or configuredto filter the power measurement to reduce noise.
 17. A method ofcontrolling a heater, the heater comprising a heating element forreceiving electrical power from a power actuator and for converting theelectrical power into heat to heat a heating surface of the heater, saidmethod comprising: receiving a temperature measurement (T_(C)) of theheating element; receiving a power measurement (P) of the electricalpower provided to the heating element; combining the temperaturemeasurement and the power measurement; and controlling the poweractuator in dependence upon the combined temperature measurement andpower measurement, the controlling comprising varying the powerdelivered to the heating element by the power actuator in dependenceupon the combined temperature measurement and power measurement to drivethe temperature of the heating surface to a desired operatingtemperature.
 18. A hair styler comprising: a heater for heating hair,the heater having a heating element for receiving electrical power andfor converting the electrical power into heat to heat a heating surfaceof the heater; a temperature sensor for sensing and outputting ameasurement of the temperature (T_(C)) of the heating element; a poweractuator for providing the electrical power to the heating element ofthe heater; a power sensor for sensing and outputting a measurement ofthe power provided to the heating element by the power actuator; andcontrol circuitry for controlling the power actuator to control thepower delivered by the power actuator to the heating element, whereinthe control circuitry is configured to: receive the temperaturemeasurement (T_(C)) from the temperature sensor; receive the powermeasurement (P) from the power sensor; combine the temperaturemeasurement and the power measurement; and vary the power delivered tothe heating element by the power actuator in dependence upon thecombined temperature measurement and power measurement to drive thetemperature of the heating surface to a desired operating temperature.